The present invention relates to optical pellicles, and more particularly to an optical pellicle packaging system.
In the semi-conductor chip industry it is well known that pattern transfer from the photomask to substrate is accomplished by exposing a mask to a light source. During the pattern transfer process, also called the photolithographic process, patterns on the photomask are projected onto the substrate which has been treated with a photosensitive substance. This results in the mask etchings being reproduced onto the substrate. Unfortunately, any foreign substance on the surface of the mask will also be reproduced on the substrate and therefore will interfere with proper pattern transfer to the substrate.
To eliminate contamination of the mask surface, a framed, thin membrane known as a pellicle is mounted on the mask surface, such that the pellicle membrane extends parallel to the mask at a predetermined distance spaced away from it. Any contamination which would ordinarily land on the mask surface instead falls onto the pellicle membrane.
Pellicles substantially eliminate the above problem because contamination on the pellicle membrane will not be projected onto the substrate. The frame of the pellicle supports the membrane at a distance spaced away from the mask surface so any particles or other contaminants on the pellicle membrane will be out of focus during pattern transfer.
The use of pellicles can increase the quality of the resulting circuit, thereby dramatically improving circuit fabrication productivity. Consequently, it is no surprise that pellicle manufacturing techniques have become increasingly important because high quality pellicles are critical to the success of the photolithographic process.
During the pellicle manufacturing process, it is important to minimize the possibility of either relatively large or small contaminant particles being deposited on the pellicle membrane. Relatively large particles are unacceptable because they may be reproduced in the substrate during photolithography even though they are out of focus. Equally unacceptable are particles (whether large or small) that are deposited on the underside of the pellicle membrane or the pellicle frame. Such particles may drop onto the mask surface during photolithography which is precisely what is to be avoided by using pellicles.
It is also critical that the pellicle membrane be extremely transparent and uniform across its surface. Because of this, pellicle membranes are normally extremely thin and are therefore quite fragile. Existing packaging systems often hold an optical pellicle in such a manner that the pellicle membrane itself is in contact with the pellicle package. This contact can damage the pellicle membrane in such a way that it is torn and is therefore not usable. It is also possible that the pellicle membrane can be damaged in the less visible fashion, but in such a way that particulate or other debris could rub off of the peripheral edges of the pellicle membrane which contact the package. This is a more dangerous type of damage, because it would not be immediately visible to the person who is involved in installing the pellicle onto a photomask. Such debris or particulate generated at the peripheral edges of the pellicle membrane could fall directly onto the more centrally disposed portions of the pellicle membrane, or worse yet, could become deposited on the photomask. In either event, there is a serious possibility that the existence of such particular could reduce the performance of the photolithography operation, event though the reason for having an optical pellicle is to improve the productivity of such operations.
One conventional packaging system such as that described above is disclosed in U.S. Pat. No. 4,697,701 to Ming Haw Ying. It can be seen in FIG. 3 of this patent that the top of the package directly contacts the peripheral portions of the optical pellicle membrane.
Thus, it is an object of the present invention to develop a pellicle packaging system which avoids the drawbacks and limitations of the prior art proposals. More specifically, the invention has as an object to develop a packaging system in which the pellicle membrane itself is not in contact with any portion of the packaging system, so to prevent any damage to the pellicle membrane during packaging, transport, and unpackaging of the pellicle. It is also an object of the present invention to develop a pellicle packaging system which is inexpensive to fabricate, which is extremely clean, and which may be used by one having minimal amount of skill, education or training.